1. Field of the Invention
The present invention relates to a cooking time control system and specifically to such a system wherein a pulse train is generated having a pulse rate proportional to the cooking rate of a certain food at the temperature of the medium in which cooking is taking place, and wherein complete cooking is detected by counting a preset number of such pulses.
2. Description of the Prior Art
Two favorite items in fast food chain restaurants are french fried potatoes and fruit turnovers. These foods are cooked in deep fat fryers, generally by placing them in a basket that is lowered into the hot fat. When cooking is complete, the basket is removed and the food either packaged immediately for sale, or placed in a holding area that is warmed by a heat source such as an infrared lamp.
Typical commercial deep fat fryers include thermostatically controlled heaters that maintain the fat or cooking oil at a nominal temperature, typically 350.degree. F. A timer may be used to control the length of time that the food is cooked, typically 21/2 minutes for french fries and 8 to 10 minutes for turnovers. A clock timer may be used for this purpose. However, this is not preferred because the time required to achieve a particular degree of crispness will very significantly with changes in the temperature of the fat during the cooking cycle. This in turn will depend on numerous factors including, among others, (1) how many potatoes or turnovers are in the basket, (2) the temperature of the food when it is first immersed in the fat, (3) whether other baskets of food are placed in the same fryer while the first batch is still cooking, (4) the initial moisture content of the food, and (5) the speed and lack of over shoot with which the heating system can bring the temperature of the fat back up to its nominal value.
By way of example, a typical commercial fryer may hold three baskets. If only one basket is placed in the hot fat, with only a few potatoes, the temperature of the oil will drop slightly and recover rapidly. However, if there are many potatoes in the basket it will take a longer time for the temperature to come back up to its nominal value.
The initial temperature and moisture content of the potatoes also will effect both how low the fat temperature will drop when the basket is inserted, and how long the temperature will remain depressed. If the potatoes are not dehydrated, but have a high water content, considerable heat energy will be expended to boil off this water, thereby keeping the fat temperature depressed for a longer period of time. If another basket of potatoes or turnovers is inserted into the fat while the first batch is frying, the temperature also will be depressed, and the same considerations will apply as to the amount and duration of temperature reduction.
The rate at which the temperature comes back up to its nominal value will depend on the characteristics of the heater and its control circuitry. In many systems the temperature may recover quickly, but then overshoot so that the fat will go to a temperature higher than the nominal value, followed by a gradual decline back to that desired temperature. All of these factors will influence the length of time that it takes to cook french fries or turnovers of desired crispness, and emphasize the fact that merely measuring the elapsed time of immersion of the food in the hot fat is not satisfctory to determine when cooking is complete.
In an attempt to overcome this problem, prior art cooking time controllers have employed temperature sensors and circuitry to adjust the cooking time dynamically to these temperature changes. However, certain shortcomings are inherent in all of these systems. Most notably, resistive temperature sensors have been used in conjunction with analog timing circuitry in which a timing capacitor was charged through a circuit dependent on the sensor resistance. This type of circuitry is not accurate for measuring a long time period, such as the 8 to 10 minutes required to cook turnovers. Even at 21/2 minutes, the typical frying time for potatoes, inaccuracy results.
Another shortcoming of prior art cooking control systems results from the change in cooking rate at different temperatures. Over large temperature ranges, the cooking rate may vary as the fourth power of temperature. Thus for deep fat frying, when the temperature is high, the cooking time is short. As the temperature decreases toward the boiling point of water, the cooking time gets extremely long. In the relatively restricted temperature range of from about 275.degree. F. to about 375.degree. F. in which deep fat frying generally is carried out, the cooking rate as a function of temperature curve can be reasonably approximated as a straight line function. However, the slope of this cooking rate vs. temperature curve usually is different from the slope of the resistance vs. temperature curve of typical resistance transducers. Thus in prior art systems which corrected the cooking time as a direct function of temperature change as sensed by a resistance probe, an error resulted because this did not compensate for change in cooking rate.
Accordingly, it is another object of the present invention to provide a cooking time control system that does compensate for changes in cooking rate as a function of temperature.
The temperature sensors used in most prior art cooking time control systems had the additional problem of being nonuniform. That is, individual sensors exhibited different change in resistance as a function of temperature. Two individual sensors, although of the same nominal specification, might exhibit different absolute resistances at a particular temperature. This compounded the problems discussed above, and led to considerable differences in corrected cooking time from system to system, for the same set of temperature change parameters of the cooking medium. To compensate for this, prior art systems were provided with an externally operated vernier time control, typically called a "crispness control". This permitted the individual restaurant operator to adjust the unit to produce a particular degree of crispness. While such control seems desirable, the actual effect may be detrimental, particularly where uniformity of product is sought. Thus one operator may set the control for a particular crispness and later a different operator may select another crispness value. This is particularly unadvantageous in fast food chain restaurants where uniformity of taste and product quality is desired in all of the chain's restaurants.
Another object of the present invention is to provide a cooking time control system which uses a platinum resistance temperature transducer, the performance of which is very uniform from device to device. A further object of the present invention is to provide a control system using such a transducer in conjunction with circuitry that results in a uniform degree of cooking without the use of an external crispness control or other vernier adjustment of the timing circuitry.
A further shortcoming of prior art control systems resulted from the use of resistance temperature sensors that exhibited a negative temperature coefficient in which the resistance decreases with increasing temperature. If such a probe should fail, an infinitely high resistance resulted. This was interpreted by the associated circuitry as being an excessively low temperature, whereas in fact the failure might have been caused by an extreme overtemperature such as when the fat came close to the flash temperature. In contrast, it is an object of the present invention to employ a positive temperature coefficient sensor. Used in conjunction with an over-temperature indicator, failure of such a transducer will be indicated as an excessive temperature condition in the cooking medium. Improved safety results.
Another object of the present invention is to provide a control system having a holding timer, started when cooking is complete and the basket is removed from the fat, for indicating how long the cooked food can be maintained in a holding area. In the case of french fries, these typically can be held for 5 to 10 minutes after they are cooked. Thereafter, they are considered no longer saleable. The holding timer gives restaurant personnel a visual or aural notification that any potatoes remaining in the holding area should be disposed of.
A further object of the invention is to allow the cooking of various items with widely varying cooking times without the use of more probes to accomplish it.